Process and plant for the production of a gas under pressure by cryogenic distillation

ABSTRACT

In a process for the production of gas under pressure by cryogenic distillation, the gas is produced by vaporizing (or pseudo-vaporizing) a liquid drawn off from a distillation column. In order to supply additional gas under pressure, a flow of a gas coming from an external source is at least partially liquefied, and the liquid thus formed is added to the liquid drawn off from the column before or after an optional pressurization step.

The present invention relates to a process and to a plant for theproduction of gas under pressure by cryogenic distillation. Inparticular, it relates to a process in which gas under pressure isproduced by vaporizing a liquid drawn off from a cryogenic distillationcolumn.

Processes of this type are well-known in the art and have existed forseveral decades.

In the present document, the pressures referred to are absolutepressures. Furthermore, the terms "condensation" and "vaporization" areintended to mean either condensation or vaporization proper, orpseudo-condensation or pseudo-vaporization, depending on whether thepressures are subcritical or supercritical.

The object of the invention is to make it possible to supply the maximumdemand for gas under pressure with an apparatus designed to produce onlya part of the liquid required for supplying the maximum gas demand.

According to the invention, a process is provided for the production ofa gas under pressure in a cryogenic separation apparatus, comprising thesteps of:

i) cooling a fluid to be separated in a heat exchanger and sending it toa distillation column of the apparatus for separation therein;

ii) drawing off a liquid flow from a column of the apparatus and heatingit in the exchanger, characterized in that

iii) at least one make-up liquid is added to the liquid flow drawn offin step ii);

iv) the mixture thus formed by the make-up liquid and the flow drawn offis heated by indirect heat exchange in an exchanger; and

v) a gas under pressure is recovered at the outlet of the apparatus.

In this way, a gas coming from an external source is used to make up forthe lack of liquid when the apparatus is operating at its maximumcapacity.

The liquefied make-up gas may have the same composition as the liquidflow drawn off.

The liquid may be an atmospheric gas. For example, liquid nitrogen maybe drawn off from the head of a single column or of a low-pressure ormedium-pressure column of a double column. Liquid argon may be obtainedat the head of an argon column. However, the invention also applies tothe separation of other cryogenic fluids; the liquid to be vaporizedcould be methane, carbon monoxide or hydrogen, for example.

Before it is vaporized, the liquid may be pressurized either byhydrostatic pressure or using a pump.

If the make-up gas is already at the vaporization pressure of the liquiddrawn off, after it has been liquefied, it may be added to the drawn-offliquid downstream of the pressurization means. Otherwise, the liquefiedmake-up gas is mixed with the liquid upstream of the pump, before beingpressurized therein.

The liquefied make-up gas preferably constitutes 20% of the vaporizedliquid flow, thus allowing the apparatus to be designed for a capacitywhich represents 80% of maximum demand.

According to the invention, a plant is also provided for the productionof a gas flow under pressure by cryogenic distillation, comprising atleast one distillation column, a heat exchanger, means for sending afluid to be separated by distillation to a distillation column, meansfor drawing off a liquid from a distillation column, and means forsending the drawn-off liquid to the heat exchanger in order to heat theliquid, characterized in that it comprises means for adding a make-upliquid to the drawn-off liquid upstream of the exchanger and means forsending the mixture thus formed to the exchanger in order to vaporize itand form the gas under pressure.

An illustrative embodiment of the invention is shown in FIG. 1, whichschematically represents a plant according to the invention.

An airflow 1 is compressed to 5.6×10⁵ kPa in a compressor, before beingdivided into three fractions. The first fraction 1A is compressed to62×10⁵ kPa by the compressor 3, refrigerated at 4 and compressed to76×10⁵ kPa. After a second refrigeration step at 6, the fraction 1A iscooled in a main exchanger 9. A part of the partially cooled air 11A isdrawn off at an intermediate temperature level from the exchanger 9 andpressure-relieved, to the pressure of a medium-pressure column 13 of adouble column 12, in a turbine 7. The pressure-relieved air is then sentinto this column 13. The remaining part of the flow 1A continues to becooled in the exchanger 9, condenses and is pressure-relieved, to thepressure of the column 13, in the valve 11 before being sent into thiscolumn.

The fraction 1B passes through the exchanger 9 before being introducedat the bottom of the column 13.

The fraction 1C is compressed to 8.9×10⁵ kPa by the compressor 15,partially cooled in the exchanger 9 and pressure-relieved, to thepressure of the low-pressure column 14, by the injection turbine 17. Thepressure-relieved fraction 1C is sent to the column 14, optionally aftera supercooling step. The injection turbine 17 drives the compressor 15.

The double column 12, comprising the low-pressure column 14 and themedium-pressure column 13, is designed to produce an average liquid flowwhich vaporizes in the exchanger 9 to form a gas under pressure. In theexample, the liquid is oxygen drawn off at a pressure of about 1.5×10⁵kPa from the bottom of the column 14 via the conduit 31. The liquid ispressurized to 76×10⁵ kPa by a pump 25, before being vaporized in theexchanger 9 to form oxygen under pressure.

Make-up oxygen gas comes from a network 19 at 30×10⁵ kPa. The make-upgas from the conduit 20 cools in the exchanger 9, is pressure-relievedthrough the valve 21 and is separated into two phases in the separator23. The gaseous part of the oxygen is sent at least in part to thelow-pressure column 14. The liquid part is sent to the conduit 31upstream or downstream of the pump 25 when the oxygen demand exceeds themaximum capacity of the double column 12, which represents 80% of themaximum demand. The liquid coming from the network is thus vaporized toform up to 20% of the maximum demand. This percentage is limited by thecapacity for liquefying the oxygen from the network acceptable to theexchanger 9.

In this way, a down-sized apparatus can nevertheless be used to supplythe entire demand for oxygen gas under pressure, with lower energycosts.

We claim:
 1. Process for the production of a gas under pressure in acryogenic separation apparatus, comprising the steps of:i) cooling in aheat exchanger a fluid to be separated and sending it to a distillationcolumn of the apparatus; ii) drawing off a liquid flow from the columnof the apparatus; iii) adding at least one make-up liquid to the liquidflow drawn off in step ii); iv) heating the mixture thus formed by themake-up liquid and the liquid flow drawn off by indirect heat exchangein the exchanger; and v) recovering a gas under pressure at an outlet ofthe apparatus.
 2. The process according to claim 1, further comprisingthe steps of cooling a make-up gas from an external source (19) in theexchanger, and at least partially condensing the cooled make-up gas toform the make-up liquid.
 3. The process according to claim 2, furthercomprising the step of cooling the make-up gas in the exchanger.
 4. Theprocess according to claim 1, in which the make-up liquid and the liquidflow drawn off have substantially the same composition.
 5. The processaccording to claim 1, in which the liquid flow drawn off is a liquidenriched in one of oxygen, nitrogen, argon and methane.
 6. The processaccording to claim 1, in which most of the gas under pressure comes froma low-pressure distillation portion of the column.
 7. The processaccording to claim 6, in which at least 80% of the gas under pressurecomes from the low-pressure column.
 8. The process according to claim 1,in which the make-up liquid is added to the liquid flow drawn offadjacent a pressurization means.
 9. The process according to claim 1, inwhich the mixture heated in step (iv) vaporizes.
 10. The processaccording to claim 1, in which the only output of the process is the gasunder pressure.
 11. An improved plant for the production of a gas flowunder pressure by cryogenic distillation, comprising at least onedistillation column (13, 14), a heat exchanger (9), means (1A, 1B, 1C)for sending a fluid to be separated by distillation to the distillationcolumn (13, 14), means (31) for drawing off a liquid from thedistillation column (14), and means for sending the drawn-off liquid tothe heat exchanger (9) in order to heat the drawn-off liquid, theimprovement comprising:means (27) for adding a make-up liquid to thedrawn-off liquid upstream of the exchanger and means for sending themixture thus formed to the exchanger (9) in order to form the gas underpressure.
 12. The plant according to claim 11, further comprising means(20) for sending a make-up gas coming from an external source to theexchanger (9) in order to cool the make-up gas, the means (21, 23) forliquefying the at least partially cooled make-up gas in order to formthe make-up liquid.
 13. The plant according to claim 11, in which thedrawn-off liquid is a liquid enriched in one of oxygen, nitrogen andargon.
 14. The plant according to claim 11, in which the distillationcolumn is a double air-distillation column and wherein the means fordrawing off a liquid are connected to a low-pressure column of thedouble air-distillation column.
 15. The plant according to claim 11, inwhich the means for sending the drawn-off liquid to the exchanger areconnected to a pressurization means (25) upstream of the exchanger. 16.The plant according to claim 15, in which the means for adding themake-up liquid to the drawn-off liquid are connected to the means forsending the drawn-off liquid to the exchanger adjacent thepressurization means (25).
 17. The plant according to claim 11, in whichthe means (1A, 1B, 1C) for sending the fluid to be separated to a columnpass at least partially through the exchanger (9).